Broad band faraday rotators



Dec. 4, 1962 R. A. wALDRoN BROAD BAND FARADAY RoTAToRs Filed Oct. '7, 1959 F/Gl W @am www ymwm 4 Unite States arent 3,067,395 BROAD BAND FARADAY ROTATORS Richard Arthur Waldron, Great Baddow, England, as-

signor to Marcons Wireless Telegraph Company Limited, London, England, a company of Great Britain Filed Oct. 7, 1959, Ser. No. 844,917 Claims priority, application Great Britain .lune 3, 1959 5 Claims. (Cl. 333-24) This invention relates to broad band Faraday rotators and more speciiically to broad band Faraday rotators of the kind operating in the so-called H11 mode of wave propagation utilising so-called Faraday rotation and cornprising a length of circularly sectioned waveguide with an inner length of coaxially positioned circularly sectioned gyro-magnetic material which is subjected to a substantially longitudinal magnetic field. Devices of this kind are well known, the gym-magnetic material being usually ferrite. The space between the inner length and the wall of the guide may be filled with any suitable dielectric, e.g. -air or a low loss solid dielectric. Devices of the kind are used for a variety of purposes, eg. in isolators or in circulators, the Faraday rotation being chosen to suit the particular purpose in each case.

The main object o-f the present invention is to provide improved devices of the kind referred to wherein the ratio of the ferrite diameter to the waveguide diameter is such that the amount of Faraday rotation shall be approximately independent of frequency over a wide range of frequencies. Theory indicates that, in practice, an approximate independence of frequency over a frequency range of well over of the said frequency is obtainable by the invention.

The invention is illustrated in and explained in connection with the `accompanying drawings in which FIG. l is an explanatory graphical figure and FIG. 2 is a simpliied part-sectional part-diagrammatic representation of one embodiment.

Before describing the present invention certain terms and quantities will iirst be defined. The rst of these is specific rotation (g55). By specific rotation is meant the actual Faraday rotation produced per unit length of device where the unit of length is the wave-length (a0) in an unbounded quantity of whatever material (commonly, though not necessarily, air) is used as the dielectric filling between the core (the ferrite or other gyromagnetic body in the guide) yand the guide. Thus if qs, is measured `in units of 1r in a length of device au we may write In this expression -l and "p12 are related to 181 and z respectively where polarisation. The relation between 1 and and between 52 and p32 is given by ice and

The other terms and quantities for preliminary definition are:

w=internal radius of the guide b=radius of the gyro-magnetic core b/w=radius ratio G/)\=the normalised guide radius e=perrnittivity of the D core material relative to permittivity of the dielectric filling material ft=the diagonal element of the relative permeability of the core material and k=the olf-diagonal element of the relative permeability of the core material the The present invention secures its object by a particular choice of the ratio b/a. Suppose, the wave-guide is empty, i.e. take the extreme theoretical case in which b/ 1:0. Then, if a/q, is large enough for wave propagation to occur, the Faraday rotation is zero. If, starting at this theoretical extreme, the ratio b/a is increased, :ps increases (assuming a/o is labove the cut off value for the empty According to this invention the radius ratio of -a microwave waveguide device of the kind referred to is at least approximately equal to that value of radius ratio at which the curve connecting specific rotation (ordinates) and radius ratio (abscissae) for the normalised guide radius at the central frequency of the intended working band of frequencies is of maximum Ordinate value. ln other words, referring to FIGURE 1, if the said central frequency is such that the specific rotation-radius curve which applies is the curve X, the radius ratio chosen is some ratio between 4ratio values such as R1 and R2 between which the slope of the curve X is small-preferably the value R where the slope is zero. In practice a choice of radius ratio within about il0% of the best value (that at which the appropriate curve has maximum ordinate value) will give acceptably good results though, obviously, the nearer the radius ratio to the best value, the better the results. As will be seen, however, owing to the comparative atness of the tops of the curves, the selection of the radius ratio is not critical. With a selection of the ratio b/a in accordance with this invention the amount of Faraday rotation, from a device of given length, is approximately independent of frequency over a useful band, i.e. the device becomes wide band.

FIGURE 2 illustrates one embodiment of the invention. In this embodiment a core in the form of a soft ferrite rod 1 of generally cylindrical shape but with pointed or tapered ends is co-axially mounted within a circularly sectioned waveguide 2, the space between the core and the guide having a dielectric filling 3 shown as of solid material. Dielectric materials other than solid-e.g. air-may be used but obviously if the dielectric is not such as will support the rod in position suitable means must be provided for doing so. At either end of the length 2 are guide lengths 4 and 5 of large enough diameter (not necessarily the same in both cases) to propafor the moment, that gate wavesof the required-'frequency when empty,v i.e. with no ferrite core. The internal radius a of the guide is so chosen with respect to the radius b of the core that the ratio b/a is at least approximately equal to the value ati:

which the curve connecting qbs and b/a for the normalised guide radius is of maximum ordinate value at the central frequency of the intended working frequency band. Thus, if the curve is the curve 'X (the curve for 'a/A?\=0.25) the Value b/a is chosen somewhere between the lines Ri and R2, preferably at R which corresponds to a radius ratio (b/a) of 0.56 and .provides a substantially constant.

value of 1.7 for rp.

For impedance matching purposes tapered guide sections 6 and 7 lead from section 4 to section 2 and from the latter section to Section while the lling 3 is tapered away outwards as shown. The tapering is dimensioned in accordance with well known principles .to avoid sharp discontinuities.

A probe 8 is exempliiied as a means for launching plane polarised H11 waves linto the guide section 4. A coaxial coil 9 provides the necessary longitudinal magnetic field for the core 1. It is energised from a D.C. source 1) through a screening switch 11 so that the direction of the applied field can be reversed and the direction of rotation as regards polarisation of the plane polarised wave which emerges at the output 12 can be reversed by changing over the switch 11.

The transverse dimensions ofthe' guide are reduced in the region of the ferrite, i.e. over guide length 2, as compared to the transverse dimensions at the end lengths 4 and 5 in order to eliminate liability for the occurrence of undesired modes of propagation.

I claim:

1. A microwave waveguide device of the kind operating in the H11 mode of wave propagation utilising Faraday rotation and comprising `a length of circularly sectioned waveguide with an inner length of coaxially positioned circularly sectioned gym-magnetic material the radius ratio b/ a being at least approximately equal to that value at which the curve connecting specic rotation os (ordi- 4 nates) and radius ratio (abscissae) for the normalised guide radius at the central frequency of the intended working band of frequencies isl of maximum ordinate value,

4,. b, aand ps being as hereindefinedrand the radius of the waveguide being such that energy will propagate in the H11 mode, and means for applying a specific longitudinal magnetic field to said length of gyro magnetic material.

2. A device Vas claimed in claim 1 wherein the value of the said radius ratio is within of that which said curve is of maximum ordinate value.

3. A device as claimed in claim 1 wherein the space between the inner wall of the waveguide and the outside of the length of gyro-magnetic material is at least partly filled with solid dielectric material.

4. A device as claimed in claiml wherein the: length of gyra-magnetic materialis in the form of a cylindrical bar length with tapered ends andthe length of waveguide is joined at both ends through tapered guide returns to further guides said length of waveguide being of smaller diameter than either of said further guides.

5. A microwave wave guide device of the kind operating in the H11-mode of'wave propagation utilisingrFaraday rotation and comprising a length of Y circularly sectioned waveguide with an inner length of coaxially positioned circularly sectioned gyra-magnetic material which is subjected to a substantially longitudinal magnetic field whereinV the radius ratio b/a is at least approximately equal to that value at which the curve connecting specic rotation p5 (ordinates) and radius ratio (abscissae) for the normal-y ised guide radius at the central frequency of the intended working band of frequencies is of zero slope, b, a and qbs being as herein defined.

Reerences Citedzin. the iile of this patent UNITED STATES PATENTS OTHER REFERENCES Fox et al.: Bell System Technical Journal, 1955, pages 22-26.

Bell publication of record, pages 27-31.

January Denis et a1. Aug. 12, 1952., 

